Various materials such as fabric from textile mills will provide a diffraction pattern from a coherent light beam passed through the material characterized principally by a central lobe and first order side lobes. In the case of fabric wherein the cross sectional area of the coherent beam passing through the fabric encompasses a large number of warp and filling threads, the developed diffraction pattern in an output plane will include a central lobe and first order side lobes along first and second axes normal to the directions of the warp and filling respectively.
In copending U.S. patent application Ser. No. 660,252 filed Feb. 23, 1976 and entitled METHOD FOR AUTOMATIC FABRIC INSPECTION, assigned to the same assignee as the present invention, there is disclosed a basic method of fabric inspection by analysis of the diffraction pattern developed from passing a coherent light beam through fabric material. In accord with this method, the height and shapes of side lobes developed in various regions of the diffraction pattern are compared to given references representative of a "good" quality of fabric. A grade count can thus be assigned to any fabric being inspected.
In my copending U.S. patent application Ser. No 660,253 filed Feb. 23, 1976 and entitled COHERENT SCANNING SYSTEM FOR FABRIC INSPECTION, also assigned to the same assignee as the present invention, there is disclosed a scanning system enabling high speed automatic inspection of large fabric areas to be carried out by the method disclosed in the first mentioned copending application. Basically, this scanning system includes a scanning mirror which, through various optical components, causes the coherent beam to scan across the width of the fabric from one edge to the other. A de-scanning mirror has directed towards it the beam as it passes through successive areas across the width of the fabric, the de-scanning mirror then directing the coherent beam to appropriate detector optics. The beam, itself is at a slight angle in a vertical plane when reflected from appropriate concave mirrors towards the fabric, the de-scanning mirror being at a lower lever than the scanning mirror. Such off-axis reflections introduce astigmatism in the imaging of the final diffraction pattern.
More particularly, the astigmatic conditions result in imaging of the diffraction pattern in the vicinity of the de-scanning mirror in first and second focal planes spaced from each other, the various lobes themselves making up the diffraction pattern being elongated. Problems are thus introduced in attempting to employ conventional techniques in detecting and analyzing the diffraction pattern.
Another problem in the form of a practical consideration involves the general physical bulk of appropriate photo-diode linear detectors for analysis of a developed diffraction pattern. If it is desired to process various regions in the diffraction pattern which regions are spaced extremely close together, it is difficult if not impossible to place individual detectors in positions to enable simultaneous processing of the different regions.
In the analysis of the differential pattern developed in a high speed automatic fabric inspection system wherein scanning is employed such as described in the heretofore referred to copending patent applications, simultaneous analysis of various regions in each successively provided diffraction pattern would be of extreme benefit insofar as overall processing time is concerned.